1. Field of the Invention
The present invention relates to a crystal oscillator and the manufacturing method thereof, the crystal oscillator preferred for use in computer clock generators, local oscillators and filters for wireless communication equipment, and the like, with a stable resonance frequency and a stable filter frequency being obtained even under conditions of ambient temperature fluctuation.
2. Description of the Related Art
Crystal oscillators which provide a stable resonance frequency during temperature changes have been used in the resonance circuitry of oscillators which generate electrical signals at a certain frequency.
FIG. 11 is a perspective view of a known crystal oscillator. FIG. 11 shows an AT-cut crystal substrate 1 which possesses an electrical axis (X) and wherein the temperature coefficient of the frequency is 0, electrode portions 2, 3 for excitation comprised of aluminum, gold, or the like, formed on both sides of the crystal substrate, and an excitation portion 4 which is the box-shaped area defined between the electrode portions 2 and 3.
This crystal oscillator is capable of generating electrical oscillation of a natural frequency within a range of around 1 kHz to 100 MHz, by means of applying a high-frequency voltage approximating a resonance frequency to the electrode portions 2 and 3.
Now, the resonance frequency of the above-described crystal oscillator possesses properties described by a curve of the third order in the event that the ambient temperature is employed as a parameter. Consequently, fluctuation in the resonance frequency is negligible when temperature fluctuation is small, but in the case where the temperature fluctuation is great, fluctuation in the resonance frequency becomes objectionably large.
Accordingly, temperature sensing devices which can provide a temperature compensating voltage to counteract this characteristic have been provided. An example is the Temperature Compensated X'tal Oscillator (TCXO) wherein the resonance frequency thereof is changed in a manner generally linear with the change in ambient temperature, by means of combining with a temperature compensating circuit which employs a thermistor described by an exponential function.
Further, generally, existence of a twin crystal within the crystal of the electronic device has adverse effects upon the properties of the device, and accordingly, it is considered to be imperative that there is no twin crystal formation within the afore-mentioned crystal. Also, it is well-known that there is no twin crystal formation within the crystal substrate used as the oscillator of the afore-mentioned crystal oscillator or the like.
However, there are various problems with the above-described crystal oscillator (TCXO), such as an increase in cost resulting from the additional electronic components required in forming the temperature compensating circuit in addition to the afore-mentioned thermistor, the complicated procedures required for adjustment of the circuit, and the like.
On the other hand, it had been thought that the resonance frequency/temperature properties of the crystal substrate described by a curve of the third order were properties inherent to the crystal substrate, with no improvement to the crystal substrate itself possible.
Accordingly, the present invention is directed to solving the afore-mentioned problems, and the object thereof is to provide a crystal oscillator wherein a stable resonance frequency and a stable filter frequency can be obtained even under conditions of ambient temperature fluctuation, by means of a relatively simple temperature compensation circuit, wherein handling is easy and no complicated adjustment is necessary, and further wherein low costs can be realized, and also to provide a method for manufacturing such a crystal oscillator.